(1) Field of the Invention
The present invention relates to an image forming apparatus and a control method, and particularly to technology of extending the lifespan of switches for turning on and off a demagnetizing coil provided in a fixing device that utilizes electromagnetic induction heating.
(2) Related Art
In recent years, fixing devices utilizing electromagnetic induction heating have been commonly adopted in electrophotographic image forming apparatuses in order to shorten the warm-up period. In fixing by electromagnetic induction heating, a fixing roller for heating and fusing toner images is heated by a magnetizing coil using induction.
When the entire length of the fixing roller in the direction of its rotation shaft is heated by the magnetizing coil using induction, if small sheets having a width smaller than the length of the fixing roller are continuously conveyed, the non-sheet passing region, where recording sheets do not pass through, is continuously heated without being cooled by the recording sheets. Consequently, the temperature of the non-sheet passing region increases. Such overheating of the non-sheet passing region leads to deterioration and damage to the fixing roller or peripheral devices, which would be a cause of a short lifespan.
A common countermeasure to the overheating of the non-sheet passing region is to use demagnetizing coils to prevent magnetic coupling of the magnetic flux, produced by the magnetizing coil, with the fixing roller. The demagnetizing coils are turned on or off according whether the non-sheet passing region is overheated or not.
In the case where a contact switch is used for turning on or off the demagnetizing coils (cf. FIG. 15), if the contact switch 1502 is turned on while the magnetizing coil 1501 is generating an alternating magnetic flux, a high voltage is applied across the contact points, and arc discharge occurs (cf. FIG. 16). Similarly, if the contact switch 1502 is turned off under such a condition, arc electrostatic discharge occurs due to the back electromotive force (cf. FIG. 17). This phenomenon causes a problem that the contact points are greatly deteriorated and fused.
Considering this problem, when a contact switch is used for turning on and off demagnetizing coils, it is necessary to temporarily stop the output of the magnetizing coil before turning on or off the contact switch, and then resume the output of the magnetizing coil. Since contact switches require a certain amount of time to complete the operation (i.e. to be completely closed or opened) after being turned on or off, it is necessary to stop the alternating current (AC) voltage provided to the magnetizing coil for at least a half period of the voltage. If the operation for fixing is continued during this period, the sheet passing region, where the recording sheets pass through, of the fixing roller decreases in temperature, and it will be impossible to maintain the image quality.
In particular, when the paper conveyance speed is high, the non-sheet passing region is likely to be overheated. Frequent switching operations of the contact switch increase the period for which the output of the magnetizing coil stops, and accordingly the decrease in temperature becomes prominent. However, if the paper conveyance speed is decreased to maintain the image quality, the efficiency in processing small sheets will be degraded.
On the other hand, a semiconductor switch (cf. FIG. 18) needs a shorter period than a contact switch to turn on or off the demagnetizing coils. Specifically, a semiconductor switch can complete the operation for turning on or off the demagnetizing coils within a short period (no longer than 1 μs) in synchronization with a zero-crossing of the alternative power provided to the magnetizing coil 1801. Therefore, the use of a semiconductor switch prevents the electrical breakdown of the switch that would be caused by high voltage. Therefore, the demagnetizing coils 1805 can be turned on or off without stopping the output from the magnetizing coil 1801, and the above-described problem caused by stopping the output from the magnetizing coil 1801 can be solved.
However, semiconductor switches have a greater on-resistance than contact switches, and therefore exhibit great loss in their conductive state, which degrades the demagnetizing efficiency. To complement the degradation of the demagnetizing efficiency, it is possible to increase the number of the turns of the demagnetizing coils. Such alteration, however, leads to the increase in size, material cost, and manufacturing cost of the demagnetizing coils. Furthermore, semiconductor switches have another problem that the loss leads to generation of heat that would deteriorate or thermally damage the semiconductor switches, and semiconductor switches are therefore unlikely to withstand for a long use.